The present invention relates to rotary torque-transmitting fluid coupling devices, and more particularly to such devices wherein the ability to dissipate heat and cool the viscous fluid represents a limiting factor on the torque-transmitting capability of the device.
Rotary fluid coupling devices of the type which may utilize the present invention have found several uses, the most common of which is to drive the cooling fan associated with the radiator of vehicle engines. Such coupling devices are frequently referred to as "viscous fan drives" because they utilize a high-viscosity fluid to transmit torque, by means of viscous shear drag, from an input coupling member (clutch) to an output coupling member (housing) to which is bolted the cooling fan.
This invention is especially advantageous when used on a relatively high-torque viscous fan drive, i.e., one which is capable of transmitting anywhere from about 2 horsepower to about 12 horsepower to the cooling fan. Typically, the clutch and the housing define a plurality of interdigitated lands and grooves which define the shear space. When this shear space is filled with viscous fluid, torque is transmitted from the clutch to the housing, in response to rotation of the clutch.
During torque transmission, substantial heat is generated as a result of the shearing of the viscous fluid between the lands and grooves. The amount of heat generated is proportional to the "slip" speed of the fan drive, i.e., the difference between the speed of the clutch and the speed of the housing. It is generally well understood by those skilled in the art that the ability to transmit torque is limited by the ability of the device to dissipate the heat generated as a result of the viscous shear. If the temperature of the viscous fluid exceeds a certain maximum temperature, the result will be a deterioration in the viscous properties of the fluid, resulting in a loss of torque-transmitting capability of the fluid.
U.S. Pat. No. 3,856,122, assigned to the assignee of the present invention, discloses a fluid coupling device in which the clutch defines a pair of arcuate channels through which fluid is pumped from the working chamber back to the reservoir chamber. However, in the coupling device of U.S. Pat. No. 3,856,122, the housing comprises the input and the clutch comprises the output. Therefore, the arcuate channels are in a location which would result in some cooling of the fluid whereas, in a conventional fluid coupling device in which the clutch is the input and the housing is the output, such channels would not be in a position relative to the flow of ram air, to provide substantial cooling.